Dietary surveys have shown that patients with Crohn's disease tend to eat more sucrose than control subjects and this investigation was undertaken to determine whether a diet rich in refined carbohydrate affects the bacterial flora of the terminal ileum. Ileostomy effluent in five patients with Crohn's disease and five with ulcerative colitis after two weeks on a diet rich in sucrose and refined cereal has been compared with the same period on a diet low in sucrose and rich in unrefined cereal. Observations were made hourly for nine hours after equicaloric breakfasts representing the two diets. The amount of ileostomy effluent was greater on the unrefined carbohydrate diet both in terms of wet weight (238 +/- 89 g vs 162 +/- 79 g, p less than 0.02) and dry weight (23 X 6 +/- 6.8 g vs 14.9 +/- 6.6 g, p less than 0.01); surprisingly, the amount of glucose and oligosaccharide was also greater (169 +/- 41 mg vs 82 +/- 26 mg, p less than 0.001) in all 10 volunteers. The bacteriological flora per gram was also higher on the unrefined carbohydrate diet after the test meal (p less than 0.02 between three and six hours) as a result of a general increase in all organisms. The relative proportions of the organisms did not vary between the two diets. No differences were detected between patients with ulcerative colitis and those with Crohn's disease.